1. Field of the Invention
This invention relates to a process for producing hydraulic cements which prevent formation of efflorescence while providing a unique cement of fancy finishes of building and which develop the same strength and surface hardness as of Portland cements.
2. Description of the Prior Art
It is already known that not only a gel (C--S--H gel) of a calcium silicate hydrate which is a main product but also calcium hydroxide [Ca(OH).sub.2 ] is produced in a process wherein a Portland cement having as a main component such calcium silicate as Tricalcium silicate (3CaO.SiO.sub.2) or dicalcium silicate (2CaO.SiO.sub.2) is kneaded with water and is then coagulated and set.
In the case of an ordinary Portland cement containing, for example, 50% by weight of tricalcium silicate (3CaO.SiO.sub.2) and 25% by weight of dicalcium silicate (2CaO.SiO.sub.2), 30 parts of calcium hydroxide [Ca(OH).sub.2 ] will be produced per 100 parts of the original cement. The calcium hydroxide frequently will appear as a white deposit called an efflorescence on the surfaces of not only various concrete products but also structures finished with plastered mortars and cement series sprayed materials. Such deposits appear as white dirty specks which spoil the beautiful appearance of the surfaces.
Such efflorescence can be divided into a primary efflorescence and a secondary efflorescence depending on the time of its generation.
The primary efflorescence is seen in the initial period of setting and will occur when calcium hydroxide [Ca(OH).sub.2 ] or the like, kneaded and dissolved in water, moves into a cement paste setting body having not yet well set and having many air gaps and is deposited on the surface. The secondary efflorescence will occur when, after the completion of the setting, rainfall or underground water is absorbed and is then carried to the surface with the drying and dissolved calcium hydroxide Ca(OH).sub.2 deposited on the surface.
Further, the efflorescence may occur due to other causes than calcium hydroxide Ca(OH).sub.2 produced by the hydrating reaction of a cement. For example, it may occur when the amount of an alkali component Na.sub.2 O or K.sub.2 O) in the cement is large, when sea water or sea sand is used for kneading water or aggregate, respectively, or when the concrete structure is partly embedded in the ground and underground water with a high inorganic salt concentration rises through the concrete structure. The composition of such efflorescence may be attributed to hydrous salt compounds such as Na.sub.2 SO.sub.4, K.sub.2 SO.sub.4, Na.sub.2 CO.sub.3, K.sub.2 CO.sub.3, CaSO.sub.4 or CaCO.sub.3.
The efflorescence of a cement or particularly the efflorescence caused by calcium hydroxide [Ca(OH).sub.2 ] is very difficult to prevent because the production of Ca(OH).sub.2 is an essential phenomenon of the hydrating reaction of a Portland cement. There has already been attempted, for example, a method wherein air gaps within a concrete are filled with a fine mineral powder to control the movement of calcium hydroxide [Ca(OH).sub.2 ] or a method wherein a material containing active silica or active alumina likely to react with Ca(OH).sub.2 is added to be subsequently fixed as an insoluble hydrate in the form of a calcium silicate hydrate (CaO--SiO.sub.2 --H.sub.2 O series) or calcium aluminate hydrate (CaO--Al.sub.2 O.sub.3 --H.sub.2 O series). However, the efflorescence can not be totally eliminated such methods.
The present invention is a process for producing hydraulic cements which can positively prevent the generation of an efflorescence by eliminating such states as are mentioned above.